Austenitic stainless steel sheet

ABSTRACT

An austenitic stainless steel sheet consists, (mass %), of: C: 0.03 to 0.15%; Si: 0.20 to 2.5%; Mn: 0.2 to 4.5%; P: 0.010 to 0.030%; S: 0.0001 to 0.0010%; Cr: 20.0 to 26.0%; Ni: 10.0 to 15.0%; Cu: 0.01 to 2.0%; Mo: 0.01 to 2.0%; Co: 0.05 to 2.50%; Al: 0.01 to 0.20%; N: 0.1 to 0.6%; V: 0.02 to 0.15%; B: 0.0002 to 0.0050%; Nb: 0 to 0.10%; Ti: 0 to 0.10%; Y: 0 to 0.10%; Ca: 0 to 0.010%; Mg: 0 to 0.010%; and REM: 0 to 0.10%, balance Fe and impurities. Contents of [Mn] and [S] satisfy [Mn]×[S]≤0.0020. Sheet thickness is 0.5 mm or smaller, an aspect ratio value L1/L2 satisfies L1/L2≥1.5 where L1 and L2 denote major and minor axis lengths of a crystal grain. A surface hardness (HV) is 300 or higher after retention at 600° C. for 400 hours.

TECHNICAL FIELD

The present invention relates to an austenitic stainless steel sheet.

BACKGROUND ART

For a gasket between a cylinder head and an engine block in an automobile engine (head gasket), stainless steels for springs, such as SUS301 and SUS403 are used. Part of SUS301 is transformed into deformation-induced martensite by cold rolling and becomes strengthened. Most part of SUS403 is transformed into martensite phase by quenching and tempering and becomes strengthened. Head gaskets are used in an environment at 200° C. or lower, and thus martensite phase exists as stabilized phase in the usage environment, exerting a sealing ability for a gasket.

Meanwhile, a gasket used between flanges that connect exhaust components of an automobile provides a connection between components that can reach a higher temperature than an engine block or a cylinder head, so that the gasket may be heated up to a temperature of 500 to 700° C. SUS301 and SUS403 have upper temperature limits of about 350° C. and cannot maintain their strengths in such a high temperature environment. Therefore, NCF625 and NCF718 defined in JIS G 4902 (Corrosion-resisting and heat-resisting superalloy plates and sheets), SUH660 and SUH310 defined in JIS G 4312 (Heat-resisting steel sheet, sheet and strip), and the like are used for gaskets for exhaust components.

NCF625 and NCF718 are alloys precipitation-strengthened by intermetallic compounds and contain 50% or more of Ni (nickel). SUH660 is a precipitation-strengthened steel containing 24 to 27% of Ni, and SUH310 is an austenitic heat resistant steel containing 19 to 22% of Ni. Containing Ni in large amount, these are all expensive. Hence, steels that contain N (nitrogen), which is also an austenite stabilization element, and contain a reduced amount of Ni have been developed and put into use.

However, recent years have increasingly seen demands for increasing fuel efficiencies of engines and exhaust emission controls for engines, and exhaust components such as EGR, turbocharger, DPF, and exhaust recovery unit are mounted even on compact ears. Downsizing of engines for increasing fuel efficiencies are progressing, and engines are aimed at miniaturization and high power, which lead to increased exhaust-gas temperatures and residual stresses generated in gaskets.

In addition, exhaust components have been downsized and additionally provided with cooling mechanisms for increased cooling efficiencies. Therefore, with a Ni-reduced heat resistant material that has been developed recently, although a strength of the material is kept while heated at high temperature, there have been cases in which stress corrosion cracking (hereafter, referred to as SCC) specific to austenitic stainless steels occurs due to condensation of exhaust gas after an engine is stopped.

Patent Document 1 discloses a steel sheet that contains increased Mn (manganese) of 1.0 to 10.0%, so that a solubility limit of N is increased, and the content of N is set at 0.35 to 0.8%, so that a high temperature strength of the steel sheet is increased through solid-solution strengthening brought by N.

Patent Document 2 discloses an austenitic stainless steel in which C—Si—N is increased, and C+2N+0.12Si+1.4Nb is adjusted to 0.45% or more, so that the austenitic stainless steel used as a metal gasket can maintain an excellent anti-sagging property when exposed to a high-temperature atmosphere. C (carbon) and N, which are elements for solid-solution strengthening of a matrix, and Si (silicon) and Nb (niobium), which inhibit movement of dislocations at the high-temperature atmosphere, are all elements that increase a high temperature strength of the austenitic stainless steel.

Patent Document 3 discloses a heat-resistant austenitic stainless steel for metal gaskets in which usage of N, as a solid-solution strengthening element, is reduced to 0.05% or less in a straighten process, so as to secure a shape flatness necessary for a gasket, and Si is used instead at more than 2.0% and 5.0% or less.

Patent Document 4 discloses a cold-rolled austenitic stainless steel, containing 7 to 25% of Ni, 16 to 30% of Cr (chromium), and 0.1 to 0.4% of N, and having a spring deflection limit of 220 MPa or higher, as a stainless steel having a stable austenite phase, inhibiting recovery and recrystallization, and being capable of occurring age hardening.

LIST OF PRIOR ART DOCUMENTS Patent Document

Patent Document 1: JP9-279315A

Patent Document 2: JP2003-82441A

Patent Document 3: JP2011-252208A

Patent Document 4: JP2012-211348A

SUMMARY OF INVENTION Technical Problem

However, the pieces of prior art described above involve the following problems.

As to the invention of Patent Document 1, while the material is excellent in strength, high temperature strength, anti-sagging property, and high-temperature oxidation resistance, no consideration is given to corrosion resistance, and thus a sufficient property cannot be obtained in corrosion resistance.

As to the steel grade described in Patent Document 2, consideration is given only to sagging property and hardness at high temperature, and a sufficient corrosion resistance cannot be obtained.

As to the invention of Patent Document 3, while a workability and an anti-sagging property at normal temperature may be improved, no consideration is given to corrosion resistance.

As to the invention Patent Document 4, consideration is given only to spring deflection limit and hardness in high-temperature use, and corrosion resistance is not considered sufficiently.

In consideration of the above, an objective of the present invention is to provide an austenitic stainless steel sheet that provides both a heat resistance suitable as a material for a metal gasket used for connecting exhaust gas channel components in an automobile engine or the like and a corrosion resistance for which no sufficient consideration has ever been given to a material for a metal gasket, and that is low in cost.

Solution to Problem

The present inventors conducted intensive studies to solve the problems described above. First, the inventors assumed that the corrosion occurring in a heat-resistant gasket was mainly SCC, based on the fact that the corrosion is likely to occur in an exhaust component that is partially cooled by a cooling device, and that the corrosion has been found in a case where residual stress occurred by a heat cycle including heating and cooling, and based on a form of the corrosion.

To decrease sensitivity to the SCC, it is effective to use a ferritic stainless steel, but the ferritic stainless steel does not provide a high temperature strength. In addition, to improvement a SCC resistance in an austenitic stainless steel, elements such as Si and Mo (molybdenum) is effectively used, but this is considered to be inappropriate because an excessive content of Si and Mo may lead to formation of a sigma phase, spoiling a fatigue life. In addition, a corrosion resistance of a stainless steel is typically expressed by Cr+3Mo+16N, as a pitting resistance equivalent number, and it is generally effective to increase Cr or N. However, it is found that Cr has a problem of forming a sigma phase, as with Si and Mo, and that an excessive content of N leads to an increase in strength at normal temperature, spoiling a producibility, and has a problem in shape fixability in press forming into a gasket shape.

Thus, influences of other elements were individually investigated in a SCC test that simulated an actual environment. As a result, the following findings were obtained as a method for improving the SCC resistance without spoiling heat resistance, workability, and press formability.

1) To improve the SCC resistance, it is effective to reduce an inclusion (MnS) that serves as a start point of occurrence of SCC. Therefore, reduction of Mn and S (sulfur) is conceivable, but Mn is also an element that increases a solubility of N. From the viewpoint of high temperature strength, N is preferably contained in a certain amount to the extent that a problem in producibility such as pore defects in solidification and edge cracking in rolling does not occur. Thus, reduction of S to 10 ppm or less (0.0010% or less) is indispensable, rather than the reduction of Mn.

2) Containing Co (cobalt) by a trace quantity does not promote precipitation of a sigma phase and is effective for corrosion resistance and high temperature strength.

The present invention has been completed based on such findings, and the gist thereof is as follows.

(1) An austenitic stainless steel sheet consisting, by mass %, of: C: 0.03 to 015%; Si: 0.20 to 2.5%; Mn: 0.2 to 4.5%; P: 0.010 to 0.030%; S: 0.0001 to 0.0010%; Cr: 20.0 to 26.0%; Ni: 10.0 to 15.0%; Cu: 0.01 to 2.0%; Mo: 0.01 to 2.0%; Co: 0.05 to 2.50%; Al: 0.01 to 0.20%; N: 0.1 to 0.6%; V: 0.02 to 0.15%; B: 0.0002 to 0.0050%; Nb: 0 to 0.10%; Ti: 0 to 0.10%; Y: 0 to 0.10%; Ca: 0 to 0.010%; Mg: 0 to 0.010%; and REM: 0 to 0.10%, the balance being Fe and impurities, wherein a content of Mn [Mn] (by mass %) and a content of S [S] (by mass %) satisfy [Mn]×[S]≤0.0020, a sheet thickness of the austenitic stainless steel sheet is 0.5 mm or smaller, an aspect ratio value L1/L2 satisfies L1/L2≥1.5 where L1 denotes a length of a major axis of a crystal grain and L2 denotes a length of a minor axis of the crystal grain, and a surface hardness (HV) is 300 or higher after retention at 600° C. for 400 hours.

(2) The austenitic stainless steel sheet according to (1), containing, by mass %, Nb: 0.01 to 0.10%, and/or Ti: 0.01 to 0.10%.

(3) The austenitic stainless steel sheet according to (1) or (2), containing one or more elements selected from, by mass %: Y: 0.01 to 0.10%; Ca: 0.001 to 0.010%; Mg: 0.0002 to 0.010%; and REM: 0.01 to 0.10%.

(4) The austenitic stainless steel sheet according to any one of (1) to (3), wherein the austenitic stainless steel sheet is subjected to a temper rolling with a rolling reduction of 20% or higher, in cold rolling.

Advantageous Effects of Invention

According to the present invention, it is possible to provide an austenitic stainless steel sheet that provides both high temperature strength and corrosion resistance with a small amount of Ni, without using 20% or more of Ni as with SUH310, SUH660, NCF625, NCF718, and the like, and the austenitic stainless steel sheet is particularly suitable for a heat-resistant metal gasket in an automobile exhaust system.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows diagrams illustrating a specimen used in Example, where FIG. 1(a) is a plan view, and FIG. 1(b) is an enlarged cross-sectional view taken along line A-A′ in FIG. 1(a).

DESCRIPTION OF EMBODIMENTS

Description will be made about a chemical composition of a stainless steel sheet according to the present invention and a preferable method for producing the steel sheet. Note that, in the following description, “%” indicating a content of each element means “mass percent”, unless otherwise specified.

<C: 0.03 to 0.15%>

C is useful for increasing a stability and a high temperature strength of an austenitic microstructure. In addition, C forms carbides with Cr, inhibiting a growth of an austenite grain to promote grain boundary oxidation moderately, so as to improve an spalling resistance for scale. To obtain this effect, the content of C is set at 0.03% or more. In addition, to inhibit the growth of the grain stably, the content of C is preferably 0.10% or more. Meanwhile, when C is contained at more than 0.15%, the amount of the Cr carbides is increased, and chromium depleted zones on a grain boundary are increased, which disable even a high-Cr austenitic stainless steel as with the present steel to keep corrosion resistance necessary for an exhaust manifold member, a turbocharger component, and the like of an automobile. Consequently, the content of C is set at 0.15% or less. From the viewpoint of corrosion resistance, the content of C is preferably 0.12% or less.

<Si: 0.20% to 2.5%>

Si is effective for oxidation resistance, particularly effective for prevention of exfoliating for scale in cyclic oxidation. In order to provide grain boundary oxidation in an environment at over 1000° C. to inhibit exfoliating for scale of a surface, 0.20% or more of Si is needed. Therefore, a content of Si is set at 0.20% or more. To increase the oxidation resistance, the content of Si is preferably 0.50% or more. Si is a ferrite stabilization element, increasing a δ ferrite amount in a solidification microstructure, which raises a problem of a decrease in hot workability in hot rolling. Therefore, the content of Si is set at 2.5% or less. Additionally, Si promotes production of a sigma phase, and embrittlement is concerned in high-temperature, long-time use. Consequently, the content of Si is preferably 2.0% or less, more preferably 1.5% or less.

<Mn: 0.2 to 4.5%>

Mn is an element used as deoxidizer and at the same time, enlarges an austenite single phase, contributing to stabilization of a steel microstructure. In addition, Mn raises a solubility limit of N, contributing to securement of a high temperature strength. The effect of the securement becomes apparent when Mn is contained at 0.2% or more. Therefore, a content of Mn is set at 0.2% or more. In addition, Mn has an effect of improving a hot workability by forming sulfides to decrease an amount of dissolved S in the steel, and therefore, the content of Mn is preferably 0.5% or more. Meanwhile, an excessive content of Mn results in a decrease in corrosion resistance. Consequently, the content of Mn is set at 4.5% or less. In addition, from the viewpoint of oxidation resistance, Cr₂O₃ is desirably predominant in oxides, and Mn oxides are not preferable. Therefore, the content of Mn is preferably 2.0% or less.

<P: 0.010 to 0.030%>

P (phosphorus) is an element included as an impurity in hot metal that is a raw material and in a main raw material such as ferrochromium. P is an element detrimental to hot workability. For that reason, a content of P is set at 0.030% or less. An excessive reduction of P leads to an increase in cost because the reduction causes indispensable use of a material of high purity. Therefore, the content of P is set at 0.010% or more. Economically, P is preferably contained at 0.020% or more.

<S: 0.0001 to 0.0010%>

S forms sulfide-based inclusions, degrading typical corrosion resistances (to general corrosion or pitting) of steel products. Therefore, an upper limit of a content of S is preferably set to be low. To reduce an inclusion (MnS), which serves as a start point of occurrence of SCC, the content of S is set at 0.0010% or less. In addition, the less the content of S, the better the corrosion resistances are. Therefore, the content of S is preferably 0.0008% or less, but to reduce S, a load of desulfurizing is increased, resulting in an increase in production cost. For that reason, the content of S is preferably set at 0.0001% or more. Generally, from the viewpoint of producing cost, the content of S is seldom set within a range of 0.0001 to 0.0010% described above. However, in the present invention, the range is set to reduce the inclusion (MnS), and it can be said that the content of S is very low.

<Cr: 20.0 to 26.0%>

Cr is an element that is essential for securing oxidation resistance and corrosion resistance in the present invention. A content of Cr less than 20.0% does not cause these effects to appear. Meanwhile, a content of Cr more than 26.0% causes an austenite single phase to decrease in size, spoiling a hot workability in production. Consequently, the content of Cr is set at 20.0 to 26.0%. From the viewpoint of oxidation resistance, the content of Cr is preferably 24.0% or more. When an amount of Cr is increased, embrittlement occurs due to formation of a sigma phase. Therefore, the content of Cr is preferably 25.0% or less.

<Ni: 10.0 to 15.0%>

Ni is an element that stabilizes an austenite phase, and is an element effective for oxidation resistance unlike Mn. These effects are obtained when Ni is contained at 10.0% or more. Therefore, a content of Ni is set at 10.0% or more. Ni also has an effect of inhibiting formation of a sigma phase, and the content of Ni is preferably 11.0% or more. Meanwhile, an excessive content of Ni leads to an increase in sensitivity to solidification cracking as well as a decrease in hot workability. For that reason, the content of Ni is set at 15.0% or less. Moreover, to inhibit scale detachment in cyclic oxidation, the content of Ni is preferably 14.0% or less.

<Cu: 0.01 to 2.0%>

Cu (copper) is a relatively inexpensive element and a substitute for Ni as an austenite stabilization element. Moreover, Cu has an effect of inhibiting propagation of crevice corrosion or pitting, and to provide the effect, Cu is preferably contained at 0.01% or more. It is noted that, in producing an austenitic stainless steel, Cu is often mixed in from a raw material such as scrap, and contained at about 0.2% as an inevitable impurity. However, a content of Cu more than 2.0% results in a decrease in hot workability, and therefore the content of Cu is set at 2.0% or less.

<Mo: 0.01 to 2.0%>

Mo is also effective for formation of protectable scale on a surface, as with Si or Cr. The effect is obtained when Mo is contained at 0.01%, and therefore, a content of Mo is set at 0.01% or more. Mo is also an element effective for improvement of corrosion resistance, and therefore, the content of Mo is preferably 0.3% or more. Meanwhile, Mo is also a ferrite stabilization element, and an increase in the content of Mo requires an increase in the content of Ni. Therefore, an excessive content of Mo is unpreferable. In addition, Mo may promote formation of a sigma phase, bringing about embrittlement. Therefore, the content of Mo is set at 2.0% or less. When the content of Mo becomes more than 0.8%, an advantageous effect of improving the corrosion resistance and the oxidation resistance is saturated. Therefore, the content of Mo is preferably 0.8% or less.

<Co: 0.05 to 2.50%>

Co is very effective for improvement of heat resistance even when contained by a trace quantity. For that reason, a content of Co is set at 0.05% or more. However, an excessive content of Co spoils hot workability, and therefore the content of Co is set at 2.50% or less. Co is an element also effective for corrosion resistance, and therefore the content of Co is preferably 0.10% or more. In addition, to inhibit formation of a sigma phase, the content of Co is preferably 2.0% or less.

<Al: 0.01 to 0.20%>

Al (aluminum) is used as a deoxidizing element and in addition, is an element that improves oxidation resistance. For that reason, a content of Al is set at 0.01% or more. To increase a deoxidation efficiency, the content of Al is preferably 0.05% or more. Meanwhile, an excessive content of Al leads to formation of its nitrides, decreasing an amount of dissolved N, which results in a decrease in high temperature strength. For that reason, the content of Al is set at 0.20% or less. In consideration of weldability at the same time, the content of Al is preferably 0.15% or less.

<N: 0.1 to 0.6%>

N is one of very important elements in the present invention. N increases a high temperature strength as with C and in addition, increases an austenite stability, enabling reduction of Ni. N has a lower influence on a decrease in corrosion resistance by sensitization than C. Therefore, N can be contained in a larger amount than C. To obtain a high temperature strength that enables endurance in high temperature environment, a content of N is set at 0.10% or more. In consideration of reduction effect of Ni at the same time, the content of N is preferably 0.25% or more. Meanwhile, containing N in a large amount causes pore defects during solidification in a steelmaking process. Therefore, the content of N is set at 0.6% or less. Moreover, N requires pressurized melting facilities and brings about an excessively high strength at normal temperature, increasing a load in cold rolling and spoiling a productivity. For that reason, the content of N is preferably set at 0.4% or less, more preferably 0.3% or less.

<V: 0.02 to 0.15%>

V (vanadium) is mixed in an alloy material of the stainless steel as an impurity and is difficult to remove in a refining process. Therefore, V is contained typically within a range of 0.02 to 0.15%. In addition, V forms fine carbonitrides to have a grain growth inhibiting effect. Therefore, V is contained intentionally as necessary. To exert the effect with stability, a content of V is set at 0.02% or more. To make a grain size within a certain range, the content of V is preferably 0.08% or more. Meanwhile, an excessive content of V may result in coarsened precipitates, so that toughness is decreased after quenching. For that reason, the content of V is set at 0.15% or less. In consideration of production cost and producibility, the content of V is preferably 0.10% or less.

<B: 0.0002 to 0.0050%>

B (boron) is an element effective for improvement of hot workability, and the effect exerts when B is contained at 0.0002% or more. Therefore, a content of B is set at 0.0002% or more. To improve hot workability in a wider temperature range, the content of B is preferably set at 0.0005% or more. Meanwhile, an excessive content of B leads to a decrease in hot workability, resulting in a surface defect. For that reason, the content of B is set at 0.0050% or less. In consideration of corrosion resistance at the same time, the content of B is preferably 0.0025% or less.

<Nb: 0 to 0.10%>

Nb is an element that forms its carbonitrides to inhibit sensitization due to precipitation of chromium carbonitrides in the stainless steel and inhibit a decrease in corrosion resistance. Therefore, Nb may be contained. However, Nb is likely to cause a surface defect by forming large steelmaking inclusions and causes a decrease in a fatigue life of a gasket. For that reason, a content of Nb is set at 0.10% or less. In consideration of improvement of high temperature strength by securing amounts of dissolved C and N, the content of Nb is preferably 0.05% or less. To obtain the effect described above, N is preferably contained at 0.01% or more.

<Ti: 0 to 0.10%>

Ti (titanium) is an element that forms its carbonitrides to inhibit sensitization due to precipitation of chromium carbonitrides in the stainless steel and inhibit a decrease in corrosion resistance. Therefore, Ti may be contained. However, Ti causes a decrease in a fatigue life of a gasket by forming large steelmaking inclusions. Therefore, the content of Ti is set at 0.10% or less. In consideration of improvement of high temperature strength by securing amounts of dissolved C and N, the content of Ti is preferably set at 0.05% or less. To obtain the effect described above, Ti is preferably contained at 0.01% or more.

<Y: 0 to 0.10%>

Y (yttrium) has an effect of increasing oxidation resistance and is, at the same time, a desulfurizing element. Therefore, Y may be contained. However, an excessive content of Y raises a problem of an anticlogging nozzle in continuous casting and spoils a fatigue life of a gasket by forming large oxide inclusions. Therefore, a content of Y is preferably 0.10% or less. To obtain the effect described above, the content of Y is preferably 0.01% or more.

<Ca: 0 to 0.010%>

Ca is used as a desulfurizing element and has an effect of reducing S in steel to improve hot workability. Therefore, Ca may be contained. Typically, Ca is added in a form of CaO in slag in melting and refining, and a part of CaO is melted in steel as Ca. In addition, Ca is contained in steel in forms of complex oxides such as CaO—SiO₂—Al₂O₃—MgO. Meanwhile, when Ca is contained in a large amount, a relatively coarse water-soluble inclusion CaS precipitates, decreasing corrosion resistance. For that reason, a content of Ca is preferably 0.010% or less. To obtain an effect of improving hot workability, the content of Ca is preferably 0.001% or more.

<Mg: 0 to 0.010%>

Mg is contained as a desulfurizing element, as with Ca. Typically, an equilibrium amount of Mg is dissolved into molten steel from slag, and in another case, Mg is contained in a form of MgO in complex oxides. In addition, MgO in a refractory material may be dissolved in molten steel. Meanwhile, an excessive content of Mg causes a water-soluble inclusion MgS to precipitate coarsely, resulting in a decrease in corrosion resistance. For that reason, a content of Mg is preferably 0.010% or less. The content of Mg is preferably 0.0002% or more.

<REM: 0 to 0.10%>

REM (rare earth metal) has an effect of increasing oxidation resistance and is, at the same time, a desulfurizing element. Therefore, REM may be contained. However, an excessive content of REM raises a problem of a nozzle anticlogging in continuous casting and spoils a fatigue life of a gasket by forming large oxide inclusions. For that reason, a content of REM is preferably 0.10% or less. To obtain the effect described above, the content of REM is preferably 0.01% or more.

REM is a generic term for Sc (scandium) and lanthanoid, and a content of REM refers to a total amount of the elements described above. Normally, REM includes Y, but in the present invention, Y is described separately.

As to the steel sheet according to the present invention, the balance consists of Fe and impurities. The term “impurities” used herein means components that are mixed in steel in producing the steel industrially, owing to various factors including raw materials such as ores and scraps, and a production process, and are allowed to be mixed in the steel within ranges in which the impurities have no adverse effect on the present invention.

<[Mn]×[5]≤0.0020>

Mn and S form a sulfide MnS, decreasing a corrosion resistance of a heat-resistant gasket in a usage environment. In particular, SCC raises a problem, and to inhibit occurrence of cracks, it is necessary to satisfy [Mn]×[S]≥0.0020, the product of a content of Mn [Mn] (in mass percent) and a content of S [S] (in mass percent). By reducing MnS serving as a start point of corrosion, it is possible to significantly inhibit occurrence of the SCC. Reduction of S leads to an increase in a desulfurizing load, and reduction of Mn leads to an increase in a production cost because of necessity of Ni in a larger amount for austenite stabilization. For that reason, a lower limit of [Mn]×[S] is preferably set at 0.0001. In consideration of workability at the same time, it is preferable to satisfy [Mn]×[S]≥0.0015.

<Aspect Ratio of Crystal Grain>

In the present invention, heat treatment is not performed after cold rolling. For that reason, a crystal grain has a rolled microstructure at a final stage. In the present invention, a length of a major axis of a crystal grain is denoted by L1, a length of a minor axis of the crystal grain is denoted by L2, and an aspect ratio L1/L2 is defined. To obtain a sufficient surface hardness, the aspect ratio: L1/L2 is set to be 1.5. When the aspect ratio: L1/L2 is <1.5, a surface hardness of the steel sheet is not sufficient for a metal gasket and, in addition, fails to satisfy a surface hardness (HV) of 300 or higher when the steel sheet is exposed at a high temperature for a long time, specifically, exposed at 600° C. for 400 hours. The aspect ratio: L1/L2 preferably satisfies L1/L2≤25 and preferably satisfies L1/L2≥5. To measure the aspect ratio, a steel microstructure is observed and recorded under an optical microscope, and subjected to image analysis, so as to be measured.

<Surface Hardness (HV) after Retention at 600° C. for 400 Hours>

The present invention is an austenitic stainless steel sheet used for a metal gasket. When used around an exhaust component, a metal gasket is normally exposed within a high-temperature range such as from 500 to 700° C., for a long time, although it depends on a part where the metal gasket is used. This exposure causes deformation to occur during its use, causing a decreasing in sealing ability and hardness, what is called “sagging”. To inhibit the sagging during use at 600° C., it is necessary to set the surface hardness (HV) of a time of exposure at 600° C. for 400 hours, at 300 or higher.

Therefore, Example to be described later simulates the usage environment described above, and surface hardness and the like are measured after retention at 600° C. for 400 hours.

The present inventors subjected steel sheets to various thermal histories, further retained the steel sheets at 600° C. for 400 hours, and measured hardnesses of the steel sheets. As a result, the present inventors confirmed that surface hardnesses (HV) were 300 or higher for all of the steel sheets. That is, even when a starting material that is actually in use, or a material a previous thermal history of which is unknown is used, this requirement of the present invention is satisfied as long as the surface hardness (HV) after the exposure at 600° C. for 400 hours, which corresponds to an assumed actual usage environment, is 300 or higher. The surface hardness (Vickers hardness) was measured by a method conforming to JIS Z 2244, with a load of 4.903 N (HV0.5) at five or more points, and an average value of the measurement was determined as a representative value.

As to the sagging, evaluation thereof is also needed in terms of deformation at high temperature. Therefore, the evaluation was made using an amount of change in bead height after the retention at 600° C. for 400 hours. The bead height refers to a height of a portion bulging into an arc shape in a cross-sectional shape, and a change in a height of the portion was measured after the retention at 600° C. for 400 hours.

<SCC Resistance>

As described above, by setting an amount ratio of Mn and S to be [Mn]×[S]≥0.0020, a SCC resistance is improved. The SCC resistance is evaluated in an autoclave test at 150° C. for 40 hours in a 0.08% NaCl aqueous solution. The autoclave test refers to a test for which a pressure-resistant container is used to obtain an aqueous solution corrosive environment at high temperature. A method of a SCC test conformed to JIS G0576, and a temperature and a composition of the solution were adjusted.

<Production Process>

In a method for producing the austenitic stainless steel according to the present invention, a process to produce a steel sheet subjected to cold temper rolling is not limited to a particular process. A steel melted by well-known means (e.g., electric furnace) is cast into a slab having a thickness of 150 to 250 mm, by a continuous casting machine, and after a surface of the slab is ground in some cases, heated to 1200° C. or higher and subjected to hot rolling by a hot rolling mill into a hot-rolled steel strip having a sheet thickness of about 3 to 6 mm. The hot-rolled steel strip is annealed at a temperature of about 1100° C. and then pickled. Subsequently, the hot-rolled steel strip is repeatedly subjected to cold rolling and annealing into a sheet having a thickness of 0.5 mm or smaller. More preferably, the thickness is 0.3 mm or smaller. Finish annealing may be annealing-pickling finish (2B finish) or BA finish in which the annealing is performed in non-oxidation atmosphere. Note that the finish annealing used herein refers to an annealing process before the temper rolling.

Meanwhile, a cold temper rolling process after the finish annealing is performed, with a rolling reduction changed in accordance with a strength (surface hardness) necessary for a spring member for a gasket so as to obtain the strength (surface hardness). To obtain a strength necessary for a heat-resistant gasket, the temper rolling is preferably performed at a rolling reduction of 20% or higher. In addition, in order for the surface hardness (HV) to satisfy 300 or higher after the retention at 600° C. for 400 hours, a composition system of the sheet is preferably set at a composition system according to the present invention, and the rolling reduction of the temper rolling is preferably set at 20% or higher, more preferably 30% or higher. An upper limit of the rolling reduction is preferably set at 80% or lower, more preferably 60% or lower because an increase in temper rolling paths spoils the productivity.

In the present invention, an annealing process is not performed after the temper rolling, a process after the temper rolling is not particularly limited as long as it is a process other than annealing. In some cases, a shape straightening process or a degreasing-cleaning process may be provided.

Example

Hereafter, effects of the present invention will be described by way of Example, but the present invention is not limited to the conditions used in the following Example.

First, steels having chemical compositions shown in Table 1 were melted and cast into slabs having a thickness of 200 mm. These slabs were heated to 1250° C., thereafter subjected to rough hot rolling and finishing hot rolling into hot-rolled steel sheets having a sheet thickness of 4 mm, inserted into heat treatment at 800° C. that simulates winding at a temperature range of 800° C., retained for one hour, and thereafter subjected to air cooling. Subsequently, the hot-rolled steel sheets were subjected to the hot-rolled sheet annealing at 1100° C. for 20 seconds and thereafter subjected to water cooling. Thereafter, the hot-rolled steel sheets were shotblasted and pickled, whereby a scale was removed. The hot-rolled steel sheets were repeatedly subjected to the cold rolling, the annealing, and the pickling several times into cold-rolled steel sheets of 0.25 to 0.5 mm. In addition, the aspect ratio was measured by the method described above.

TABLE 1 Test steel Chemical Composition (mass %, Balance: Fe and Impurities) No. C Si Mn P S Cr Ni Mo Cu Co Ti Nb V Al N B Y Cs Mg REM Mn × S A1 0.09 2.00 1.50 0.025 0.0002 24.2 12.1 0.50 0.20 0.20 0.01 0.02 0.08 0.03 0.23 0.0002 — — — — 0.0003 A2 0.07 0.30 0.50 0.024 0.0003 23.2 11.0 0.01 0.05 0.15 0.01 0.02 0.07 0.01 0.20 0.0003 — — — — 0.0002 A3 0.03 1.20 1.30 0.025 0.0007 24.4 11.9 0.20 1.00 0.10 0.03 0.05 0.03 0.06 0.35 0.0042 — — — — 0.0009 A4 0.15 0.80 1.40 0.018 0.0007 24.0 14.9 0.20 0.89 0.12 0.01 0.04 0.06 0.14 0.10 0.0032 — — — — 0.0010 A5 0.15 0.90 4.50 0.029 0.0002 25.6 10.6 0.30 0.31 0.15 0.03 0.04 0.09 0.05 0.25 0.0009 — — — — 0.0009 A6 0.05 1.28 1.23 0.020 0.0010 22.8 10.1 0.10 0.10 0.14 0.01 0.02 0.06 0.05 0.35 0.0009 — — — — 0.0012 A7 0.03 1.20 1.01 0.026 0.0001 26.0 10.0 0.20 0.20 0.20 0.01 0.03 0.03 0.07 0.53 0.0037 — — — 0.0100 0.0001 A8 0.11 0.20 3.31 0.022 0.0005 24.0 14.4 2.00 0.20 0.21 0.03 0.05 0.10 0.13 0.15 0.0050 — — — 0.1000 0.0017 A9 0.05 0.60 1.77 0.030 0.0009 21.8 11.3 0.20 0.20 0.22 0.01 0.003 0.10 0.20 0.19 0.0037 — — — — 0.0015 A10 0.04 1.60 2.00 0.024 0.0010 20.7 11.0 0.10 0.20 0.15 0.04 0.002 0.11 0.06 0.24 0.0014 — — — — 0.0020 A11 0.14 0.76 0.20 0.028 0.0009 25.4 14.2 0.20 0.20 0.15 0.04 0.02 0.13 0.12 0.30 0.0042 — — — — 0.0002 A12 0.07 2.50 1.20 0.026 0.0002 23.0 10.9 0.10 0.30 0.10 0.05 0.002 0.11 0.13 0.42 0.0042 — — — — 0.0002 A13 0.09 1.70 1.20 0.028 0.0006 23.8 10.4 0.20 0.30 0.05 0.02 0.02 0.04 0.18 0.48 0.0042 — — — — 0.0007 A14 0.11 1.50 1.00 0.024 0.0004 22.7 14.4 0.10 0.20 0.30 0.04 0.10 0.06 0.11 0.28 0.0012 — — — — 0.0004 A15 0.03 1.50 0.80 0.010 0.0009 25.0 10.0 0.10 0.12 0.10 0.02 0.04 0.05 0.17 0.50 0.0037 — — — — 0.0007 A16 0.11 0.50 0.66 0.016 0.0006 24.4 11.5 0.20 2.00 0.05 0.003 0.04 0.09 0.03 0.12 0.0031 0.0100 0.0050 0.0010 — 0.0004 A17 0.12 0.40 3.68 0.012 0.0002 21.3 10.5 0.10 0.30 0.40 0.01 0.03 0.12 0.19 0.13 0.0044 0.1000 — — — 0.0007 A18 0.15 0.30 1.90 0.020 0.0002 24.0 13.2 0.05 0.30 0.21 0.01 0.04 0.09 0.11 0.32 0.0016 — — — — 0.0004 A19 0.09 0.84 0.34 0.020 0.0009 25.0 10.0 0.03 0.10 0.18 0.10 0.01 0.15 0.06 0.39 0.0026 — — — — 0.0003 A20 0.04 0.63 3.00 0.030 0.0005 20.0 15.0 0.10 0.10 2.50 0.01 0.02 0.13 0.19 0.15 0.0024 — — — — 0.0014 A21 0.07 0.75 3.52 0.015 0.0001 23.8 10.9 0.07 0.10 0.22 0.01 0.02 0.15 0.07 0.20 0.0046 — — — — 0.0004 A22 0.14 0.78 1.57 0.024 0.0005 21.2 10.4 0.10 0.01 0.21 0.03 0.04 0.10 0.04 0.20 0.0013 — — — — 0.0008 A23 0.05 0.20 1.94 0.012 0.0003 24.0 13.3 0.20 0.20 0.20 0.10 0.004 0.03 0.20 0.21 0.0019 — — — — 0.0012 B1 0.02 * 1.96 1.50 0.026 0.0010 21.0 11.0 0.20 0.20 0.30 0.01 0.01 0.10 0.03 0.23 0.0001 * — — — — 0.0015 B2 0.20 * 1.83 0.50 0.019 0.0009 24.0 10.0 0.54 0.68 0.10 0.04 0.04 0.08 0.19 0.22 0.0037 — — — — 0.0004 B3 0.15 0.10 * 3.37 0.030 0.0006 25.6 11.5 1.61 0.30 0.06 0.0016 0.04 0.12 0.19 0.24 0.0060 * — — — — 0.0021 * B4 0.07 2.60 * 0.50 0.030 0.0002 23.5 10.0 0.22 2.10 * 0.20 0.04 0.04 0.10 0.19 0.30 0.0028 — — — — 0.0001 B5 0.04 1.61 0.10 * 0.014 0.0003 23.0 14.8 1.19 0.25 0.38 0.01 0.05 0.08 0.19 0.23 0.0009 — — — — 0.00003 B6 0.05 1.29 4.60 * 0.019 0.0001 22.8 12.2 1.83 1.19 0.20 0.03 0.05 0.12 0.03 0.21 0.0032 — — — — 0.0006 B7 0.13 0.31 0.30 0.005 * 0.0004 26.0 13.1 1.63 0.76 0.12 0.03 0.04 0.03 0.30 * 0.19 0.0002 — — — — 0.0001 B8 0.08 1.46 0.51 0.040 * 0.0007 22.2 12.0 0.72 1.53 0.30 0.03 0.04 0.06 0.13 0.18 0.0031 — — — — 0.0004 B9 0.03 1.27 2.56 0.027 — * 21.3 11.0 1.53 0.07 0.42 0.05 0.05 0.06 0.30 * 0.05 * 0.0014 — — — — — B10 0.11 0.58 2.81 0.020 0.0020 * 25.4 14.8 0.61 1.67 0.14 0.03 0.01 0.08 0.14 0.05 * 0.0037 — — — — 0.0056 * B11 0.07 3.00 * 0.58 0.013 0.0008 18.0 * 10.4 1.20 0.49 0.20 0.03 0.003 0.11 0.18 0.21 0.0010 — — — — 0.0004 B12 0.21 * 1.50 1.38 0.015 0.0004 27.0 * 10.5 0.47 0.51 0.32 0.04 0.0001 0.15 0.19 0.70 * 0.0031 — — — — 0.0005 B13 0.14 1.03 3.93 0.013 0.0006 21.1  9.0 * 0.72 1.30 0.36 0.03 0.03 0.12 0.13 0.18 0.0012 — — — — 0.0022 * B14 0.07 1.94 2.89 0.030 0.0010 23.7 16.0 * 1.24 1.87 0.21 0.03 0.05 0.15 0.05 0.17 0.0033 — — — — 0.0029 * B15 0.12 0.89 1.39 0.014 0.0002 24.2 11.6 — * 1.68 1.16 0.01 0.04 0.20 * 0.05 0.20 0.0030 — — — — 0.0002 B16 0.11 1.13 3.38 0.011 0.0003 22.6 14.0 2.50 * 1.84 0.21 0.00002 0.04 0.18 * 0.15 0.19 0.0031 — — — — 0.0009 B17 0.13 1.44 2.47 0.021 0.0004 20.9 11.7 1.01 — * 0.21 0.02 0.05 0.13 — * 0.21 0.0033 — — — — 0.0010 B18 0.07 0.91 0.57 0.030 0.0001 21.2 11.9 1.74 2.20 * 0.20 0.01 0.01 0.13 0.17 0.11 0.0046 — — — — 0.0001 B19 0.10 0.90 0.56 0.021 0.0002 25.7 14.3 0.32 1.51 0.04 * 0.04 0.01 0.07 0.16 0.21 0.0039 — — — — 0.0001 B20 0.08 1.97 1.88 0.013 0.0002 20.0 11.2 1.53 0.31 3.00 * 0.04 0.02 0.02 0.02 0.23 0.0012 — — — — 0.0004 B21 0.09 1.62 1.05 0.018 0.0005 22.8 11.0 0.72 1.39 0.30 0.15 * 0.04 0.05 0.18 0.27 0.0011 — — — — 0.0005 B22 0.04 0.44 3.49 0.026 0.0009 24.6 13.5 1.65 0.19 0.32 0.05 0.15 * 0.14 0.16 0.25 0.0008 — — — — 0.0032 * B23 0.07 0.75 2.20 0.015 0.0010 23.8 10.9 0.07 0.10 0.22 0.01 0.02 0.15 0.07 0.20 0.0046 — — — — 0.0022 * The mark “*” indicates that the chemical composition fell out of the range defined in the present invention.

As illustrated in FIG. 1, from each stainless steel sheet 1, a specimen 10 was fabricated, the specimen 10 including a circular opening 2 having an inner diameter of 80 mm and a bead 3 that is made around the opening 2 by press forming to have a width of 2.5 mm, a height of 0.25 mm, and a protrusion of 2R and that simulated a metal gasket. The specimen 10 was retained at 600° C. for 400 hours and thereafter a surface hardness of the specimen was measured. Measurement of the surface hardness was performed by a method conforming to JIS Z 2244, with a load of 4.903 N (HV0.5) at five or more points, and an average value of the measurement was determined as a representative value.

In addition, the change in bead height was measured and evaluated as a gasket sagging. When the change in bead height of the specimen 10 was 30% or smaller, the specimen 10 was accepted. In addition, the SCC was evaluated in the autoclave test at 150° C. for 40 hours in the 0.08% NaCl aqueous solution. As comparative examples, samples having compositions falling out of those of the present invention were subjected to similar evaluation. The method of a SCC test conformed to JIS G0576, and a temperature and a composition of the solution were adjusted.

The results of the evaluation are shown in Table 2. Note that a rolling reduction of the cold rolling performed the last (the temper rolling) is shown as a “temper rolling reduction”.

TABLE 2 Surface hardness Test Temper Annealing Aspect (HV) after Test steel rolling after temer ratio retention at 600° C. Occurrence Change in No. No. reduction rolling (L1/L2) for 400 h of SCC bead height Remarks  1 A1 60 Absent 6.3 420 No corrosion 30% or smaller Inventive  2 A2 60 Absent 6.3 490 No corrosion 30% or smaller example  3 A3 60 Absent 6.3 484 No corrosion 30% or smaller  4 A4 60 Absent 6.3 420 No corrosion 30% or smaller  5 A5 60 Absent 6.3 402 No corrosion 30% or smaller  6 A6 60 Absent 6.3 406 No corrosion 30% or smaller  7 A7 60 Absent 6.3 554 No corrosion 30% or smaller  8 A8 60 Absent 6.3 420 No corrosion 30% or smaller  9 A9 60 Absent 6.3 390 No corrosion 30% or smaller 10 A10 60 Absent 6.3 410 No corrosion 30% or smaller 11 A11 60 Absent 6.3 431 No corrosion 30% or smaller 12 A12 60 Absent 6.3 497 No corrosion 30% or smaller 13 A13 60 Absent 6.3 571 No corrosion 30% or smaller 14 A14 60 Absent 6.3 393 No corrosion 30% or smaller 15 A15 60 Absent 6.3 450 No corrosion 30% or smaller 16 A16 60 Absent 6.3 380 No corrosion 30% or smaller 17 A17 60 Absent 6.3 450 No corrosion 30% or smaller 18 A18 60 Absent 6.3 400 No corrosion 30% or smaller 19 A19 60 Absent 6.3 450 No corrosion 30% or smaller 20 A20 60 Absent 6.3 420 No corrosion 30% or smaller 21 A21 60 Absent 6.3 380 No corrosion 30% or smaller 22 A22 60 Absent 6.3 380 No corrosion 30% or smaller 23 A23 60 Absent 6.3 390 No corrosion 30% or smaller 24 A2 20 Absent 1.5 320 No corrosion 30% or smaller 25 A2 40 Absent 2.8 400 No corrosion 30% or smaller 26 B1 * 60 Absent 6.3 280 * No corrosion 40% Surface defect Comparative 27 B2 * 60 Absent 6.3 420 SCC occurred 30% or smaller example 28 B3 * 60 Absent 6.3 391 SCC occurred 30% or smaller Surface defect 29 B4 * 60 Absent 6.3 369 No corrosion 45% 30 B5 * 60 Absent 6.3 320 No corrosion 30% or smaller Surface defect 31 B6 * 60 Absent 6.3 320 SCC occurred 30% or smaller 32 B7 * 60 Absent 6.3 324 No corrosion 40% 33 B8 * 60 Absent 6.3 380 No corrosion 30% or smaller Surface defect 34 B9 * 60 Absent 6.3 250 * No corrosion 42% 35 B10 * 60 Absent 6.3 350 SCC occurred 30% or smaller 36 B11 * 60 Absent 6.3 380 SCC occurred 30% or smaller 37 B12 * 60 Absent 6.3 530 No corrosion 75% 38 B13 * 60 Absent 6.3 316 SCC occurred 30% or smaller Surface defect 39 B14 * 60 Absent 6.3 390 SCC occurred 30% or smaller Surface defect 40 B15 * 60 Absent 6.3 420 No corrosion 60% 41 B16 * 60 Absent 6.3 405 No corrosion 56% 42 B17 * 60 Absent 6.3 338 No corrosion 30% or smaller Surface defect 43 B18 * 60 Absent 6.3 320 No corrosion 30% or smaller Surface defect 44 B19 * 60 Absent 6.3 250 * No corrosion 50% 45 B20 * 60 Absent 6.3 312 No corrosion 56% 46 B21 * 60 Absent 6.3 361 No corrosion 70% 47 B22 * 60 Absent 6.3 310 SCC occurred 65% 48 B21 * 60 900° C., 20 s 1.0 * 200 * No corrosion 80% 49 B23 * 16 950° C., 20 s 1.4 * 280 * SCC occurred 60% 50 B23 * 55 Absent 5.0 320 SCC occurred 30% or smaller 51 A2 16 950° C., 20 s 1.4 * 220 * No corrosion 70% The mark “*” indicates that the value fell out of the range defined in the present invention.

Test Nos. 1 to 25, which were example embodiments of the present invention, satisfied the definition of the composition according to the present invention, satisfied [Mn]×[S]≤0.0020, satisfied the aspect ratio L1/L2≥1.5, and showed surface hardnesses (HV) of 300 or higher after the retention at 600° C. for 400 hours. In Test Nos. 1 to 25, the change in bead height satisfied a target value (30% or smaller), and neither SCC nor surface defect occurred.

Meanwhile, Test Nos. 26 to 47, which were comparative examples, did not satisfy the definition of the composition according to the present invention, and showed surface hardnesses lower than 300 after the retention at 600° C. for 400 hours. In Test Nos. 26 to 47, one or more than one of the SCC, a change in bead height exceeding 30%, and the surface defect occurred.

Test No. 48, which was a comparative example, did not satisfy the definition of the composition according to the present invention, was low in aspect ratio value, and showed a low value of surface hardness (HV) after the retention at 600° C. for 400 hours. In addition, in Test No. 48, the change in bead height exceeded 30%.

In Test No. 49, which was a comparative example, although the composition of the elements fell within the ranges defined in the present invention, [Mn]×[S]≤0.0020 was not satisfied, the aspect ratio was low too, and the surface hardness (HV) after the retention at 600° C. for 400 hours was a low value. In addition, the SCC occurred, and the change in bead height exceeded 30%.

In Test No. 50, which was a comparative example, although the composition of the elements fell within the ranges defined in the present invention, [Mn]×[S]≤0.0020 was not satisfied, and the SCC occurred.

Test No. 51, which was a comparative example, satisfied the definition of the composition according to the present invention but was low in aspect ratio value and showed a low value of the surface hardness (HV) after the retention at 600° C. for 400 hours, and the change in bead height exceeded 30%.

REFERENCE SIGNS LIST

-   -   1 stainless steel sheet     -   2 opening     -   3 bead     -   10 specimen 

1. An austenitic stainless steel sheet consisting, by mass %, of: C: 0.03 to 0.15%; Si: 0.20 to 2.5%; Mn: 0.2 to 4.5%; P: 0.010 to 0.030%; S: 0.0001 to 0.0010%; Cr: 20.0 to 26.0%; Ni: 10.0 to 15.0%; Cu: 0.01 to 2.0%; Mo: 0.01 to 2.0%; Co: 0.05 to 2.50%; Al: 0.01 to 0.20%; N: 0.1 to 0.6%; V: 0.02 to 0.15%; B: 0.0002 to 0.0050%; Nb: 0 to 0.10%; Ti: 0 to 0.10%; Y: 0 to 0.10%; Ca: 0 to 0.010%; Mg: 0 to 0.010%; and REM: 0 to 0.10%, the balance being Fe and impurities, wherein a content of Mn [Mn] (by mass %) and a content of S [S] (by mass %) satisfy [Mn]×[S]≤0.0020, a sheet thickness of the austenitic stainless steel sheet is 0.5 mm or smaller, an aspect ratio value L1/L2 satisfies L1/L2≥1.5 where L1 denotes a length of a major axis of a crystal grain and L2 denotes a length of a minor axis of the crystal grain, and a surface hardness (HV) is 300 or higher after retention at 600° C. for 400 hours.
 2. The austenitic stainless steel sheet according to claim 1, containing, by mass %, Nb: 0.01 to 0.10%, and/or Ti: 0.01 to 0.10%.
 3. The austenitic stainless steel sheet according to claim 1, containing one or more elements selected from, by mass %: Y: 0.01 to 0.10%; Ca: 0.001 to 0.010%; Mg: 0.0002 to 0.010%; and REM: 0.01 to 0.10%.
 4. The austenitic stainless steel sheet according to claim 1, wherein the austenitic stainless steel sheet is subjected to a temper rolling with a rolling reduction of 20% or higher, in cold rolling.
 5. The austenitic stainless steel sheet according to claim 2, containing one or more elements selected from, by mass %: Y: 0.01 to 0.10%; Ca: 0.001 to 0.010%; Mg: 0.0002 to 0.010%; and REM: 0.01 to 0.10%.
 6. The austenitic stainless steel sheet according to claim 2, wherein the austenitic stainless steel sheet is subjected to a temper rolling with a rolling reduction of 20% or higher, in cold rolling.
 7. The austenitic stainless steel sheet according to claim 3, wherein the austenitic stainless steel sheet is subjected to a temper rolling with a rolling reduction of 20% or higher, in cold rolling.
 8. The austenitic stainless steel sheet according to claim 5, wherein the austenitic stainless steel sheet is subjected to a temper rolling with a rolling reduction of 20% or higher, in cold rolling. 